1. Field of the Invention
The present invention relates to a rod lens array and an equal-magnification imaging optical apparatus using a rod lens array, and more particularly, to a rod lens array used for reading an image in an image reading apparatus and an equal-magnification imaging optical apparatus using a rod lens array.
2. Description of the Related Art
In the related art, in an image reading apparatus such as a scanner or a facsimile, a technique of allowing light emitted from a light source to irradiate a document and allowing a CCD sensor to receive the reflected light through a rod lens array is widely used.
In recent years, it is desired to improve a document reading accuracy of the image reading apparatus, and as one of means for achieving the improvement, improving of resolution (MTF: Modulation Transfer Function) of the rod lens array is performed. In addition, since the reading accuracy of the rod lens array depends on an overlap degree (m value) and unevenness in the quantity of light of an image transmitted by a number of rod lenses arranged, a technique of improving the reading accuracy of the rod lens array by setting these values to appropriate values is known (for example, Japanese Patent Laid-Open No. 11-64605).
If the overlap degree m exceeds 2.5, brightness of the image is decreased. Therefore, Japanese Patent Laid-Open No. 11-64605 discloses a technique of suppressing the light quantity unevenness by setting the overlap degree of rod lens to be in a range of 1.61 to 1.80 or a range of 2.06 to 2.50.
However, although the overlap degree is set in the range disclosed in Japanese Patent Laid-Open No. 11-64605, the light quantity unevenness may not be sufficiently suppressed. Practically, in order to prevent deterioration in image caused by the light quantity unevenness, it is necessary to perform correction using electric light quantity for allowing the entire image to have uniform brightness or to perform shading correction for correcting sensitivity of a sensor. Particularly, when an optical apparatus such as a facsimile, a copier, a printer, and a scanner is to be assembled, assembly error occurs between a surface of a document and an end surface of a rod lens array or between a surface of a sensor and the end surface of the rod lens array. In addition, the performance of a rod lens is easily influenced by aged deterioration or use environment. Furthermore, even in the case where a housing is slightly deformed due to vibration during the transportation of a highly-accurate optical member using the rod lens array, the performance of the rod lens is changed.
Therefore, although the light quantity unevenness is corrected before manufacturing the rod lens array, the correction needs to be performed again at the site of the assembly of an actual optical apparatus such as a facsimile, a copier, a printer, and a scanner.